[css-fonts-5] Broaden language about @font-face metrics descriptors' normal value

The original text was overly specific. In particular, `normal` values don't always
need to come directly from the font file. `normal` is intended to just do the same
thing as what happens today, without any of these metrics descriptors.

Both editors agree that this is just an editorial fix, and can just be committed
without an issue or WG discussion.

Author: litherum

css-fonts-5/Overview.bs

@@ -172,8 +172,8 @@ The 'ascent-override', 'descent-override' and 'line-gap-override' descriptors de
 <a spec="CSS-INLINE-3">ascent metric</a>, <a spec="CSS-INLINE-3">descent metric</a> and
 <a spec="CSS-INLINE-3">line gap metric</a> of the font, respectively.
 
-When the descriptor value is 'normal', the corresponding metric value is obtained from the
-font file directly.
+When the descriptor value is 'normal', the corresponding metric value is obtained as-if the
+descriptor was absent from the <code>@font-face</code> block.
 
 Note: User agents may draw data from different places from the font file as the metric values,
 which results in different text layouts.